Jeep model NP231 transfer cases have one glaring weak point. They use a slip yoke on the rear output shaft of the transfer case instead of having a fixed yoke and a drive shaft with slip splines.\ \ This design works fine on the street but once you have lifted your Jeep and installed bigger tires on it, the stress on the slip yoke or rear output shaft is often too much. This Heavy Duty Slip Yoke eliminator kit includes everything to modify your transfer case, this kit does require our extended travel drive shaft, which helps to prevent driveline vibration and the rear ouput shaft from breaking.